Raw material pitch for carbon fiber production

ABSTRACT

A raw material pitch for carbon fiber production, with which carbon fiber excellent in tensile strength can be produced at relatively low costs, is provided. The raw material pitch is obtained from coal, is a pitch for producing carbon fiber by melt spinning, and contains oxygen in a content of 1.0% by mass or more and a toluene-soluble component in a content of 20% by mass or more. The coal is preferably bituminous coal or sub-bituminous coal. The raw material pitch is preferably prepared by subjecting a solvent-soluble component separated from a thermal decomposition product of coal in a solvent by a solvent extraction treatment at a temperature of less than 300° C. to a heat treatment at a temperature of 150° C. or more.

TECHNICAL FIELD

The present invention relates to raw material pitch for carbon fiber production.

BACKGROUND ART

Carbon fiber has been widely utilized, for example, as a reinforcement material for structural materials such as resin, concrete, and ceramic. Moreover, separately, for example, carbon fiber is also utilized as a heat insulating material, a raw material of activated carbon, an electroconductive material, a heat conductive material, or the like.

In general, a synthetic resin such as polyacrylonitrile or a kind of pitch obtained from petroleum or coal is spun into yarn to be formed in a fiber-form and this yarn is subjected to infusibilization (air oxidation) and carbonization to produce carbon fiber. Among the above-mentioned raw materials, coal pitch is a residue left after distilling coal tar, which is a liquid substance secondarily produced at the time of dry-distilling coal to produce coke, to remove volatile components such as naphthalene and is a viscous black substance. Such coal pitch is a mixture of many kinds of compounds largely containing an aromatic compound containing a large number of benzene rings in its skeleton.

More specifically, since coal pitch is heated to 1000° C. or so on production of coke, the coal pitch is composed mainly of a polycyclic aromatic compound being high in the ring-condensation degree, and for example, the coal pitch has an extremely small content ratio of the alkyl-side chain such as a methyl group, an ethyl group, and a propyl group and an extremely small content ratio of the oxygen-containing structure such as an ether bond and a phenol group. As an index for the content ratio of such structure, an oxygen content ratio can be used and, then, the oxygen content ratio of coal pitch is 1% by mass or less in general and 0.5% by mass or less in many cases.

Since such coal pitch melts into a viscous liquid when heated to 100° C. to 200° C. or so, this can be extruded from a nozzle to be spun into yarn. However, for example, coal pitch contains various components such as metal impurities and a solid-state carbon component, by which spinning and subsequent infusibilization and carbonization are inhibited, because the coal pitch is a by-product on production of coke and is a substance recovered as a residue, as described above. On this account, it is difficult to stably and efficiently produce carbon fiber from coal pitch. Moreover, these impurities can cause the carbon fiber produced to have defects and can cause the tensile strength of the resulting carbon fiber to be lowered.

Moreover, it is preferred that raw material pitch, which is used in the production of carbon fiber, uniformly melts at a constant temperature on spinning. Moreover, the softening point of raw material pitch is preferably 150° C. or more so that the temperature of an infusibilization treatment for the shape fixation of fiber obtained by spinning the raw material pitch into yarn can be increased and the treatment can be streamlined and is preferably 350° C. or less so that the raw material pitch can be spun into yarn at a temperature causing no thermal decomposition reaction on spinning.

In order to make raw material pitch satisfy these requirements, there has been proposed a technique for subjecting coal to a solvent extraction treatment to obtain pitch, and for example, subjecting the pitch to treatments such as a component adjustment and a removal of impurities to reform coal pitch (for example, see JP H07-15099 B).

However, the reforming treatment of such coal pitch mentioned above becomes a factor that pushes up the production cost of carbon fiber.

RELATED ART DOCUMENT Patent Document

-   Patent Document 1: JP H07-15099 B

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

The present invention has been made in view of the above-mentioned inexpediency and an object thereof as a problem to be solved is to provide raw material pitch for carbon fiber production with which carbon fiber excellent in tensile strength can be produced at relatively low costs.

Means for Solving the Problems

The present invention has been made in view of solving the above-mentioned problem and is directed to raw material pitch for carbon fiber production, being obtained from coal, being a kind of pitch for producing carbon fiber by melt spinning, and having a content ratio of oxygen of 1.0% by mass or more and a content ratio of a toluene-soluble content of 20% by mass or more.

Since the raw material pitch for carbon fiber production has a content ratio of oxygen of 1.0% by mass or more, in a carbonization process, an oxygen atom forms a crosslinkage between molecules. On this account, the raw material pitch for carbon fiber production can inhibit stacked aromatic rings from being formed to suppress the growth of crystals. With this setup, the tensile strength of the resulting carbon fiber is enhanced because, when a stress acts on carbon fiber, the stress concentration on a portion between microcrystals can be relaxed. Moreover, since the raw material pitch for carbon fiber production has a content ratio of a toluene-soluble content, which is constituted of compounds with a relatively small molecular weight, of 20% by mass or more, the raw material pitch is excellent in meltability and spinnability to be exerted in a melt spinning process. Accordingly, by using the raw material pitch for carbon fiber production, carbon fiber excellent in tensile strength can be produced at relatively low costs.

The above-mentioned coal is preferably bituminous coal or sub-bituminous coal. As such, when the above-mentioned coal is bituminous coal or sub-bituminous coal, since the yield of the raw material pitch for carbon fiber production becomes relatively high, as a result, not only the raw material pitch for carbon fiber production but also carbon fiber can be produced at relatively low costs.

The raw material pitch for carbon fiber production is preferably prepared by subjecting a solvent-soluble component separated from a thermal decomposition product of coal in a solvent by a solvent extraction treatment at a temperature of less than 300° C. to a heat treatment at a temperature of 150° C. or more. As such, when raw material pitch for carbon fiber production is prepared by subjecting a solvent-soluble component separated from a thermal decomposition product of coal in a solvent by a solvent extraction treatment at a temperature of less than 300° C. to a heat treatment at a temperature of 150° C. or more, the content ratio of oxygen and the content ratio of a toluene-soluble content can be easily and certainly made to lie within the above-mentioned range. Consequently, not only the raw material pitch for carbon fiber production but also carbon fiber excellent in tensile strength can be produced at relatively low costs.

In this context, “the content ratio of oxygen” means a content ratio of the oxygen atom including not only an oxygen molecule but also an oxygen atom bonded to another atom, and specifically, means a value measured in accordance with JIS-M8813 (2004). Moreover, “the content ratio of a toluene-soluble content” refers to a value measured in accordance with JIS-K2207 (1996). Moreover, “bituminous coal” and “sub-bituminous coal” refer to the respective kinds of coal having a coal quality stipulated in JTS-M1002 (1978).

Effect of the Invention

As stated above, by using the raw material pitch for carbon fiber production of the present invention, carbon fiber can be produced at low costs.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a flowchart showing the procedure of a method of producing raw material pitch for carbon fiber production in accordance with one embodiment of the present invention.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described in detail appropriately with reference to the drawing.

[Raw Material Pitch for Carbon Fiber Production]

The raw material pitch for carbon fiber production in accordance with one embodiment of the present invention is obtained from coal and is a kind of raw material pitch for producing carbon fiber by melt spinning.

It is preferred that the raw material pitch for carbon fiber production be a kind of pitch obtained from a thermal decomposition product of coal in a solvent. The coal includes a large number of structural units containing an oxygen atom such as an alkyl side chain and largely contains a toluene-soluble content as compared with coal tar and a residue in the petroleum producing process, which have been treated at a relatively high temperature. On this account, the raw material pitch for carbon fiber production obtained from a thermal decomposition product of coal in a solvent can be provided with a feature described below.

The lower limit of the content ratio of oxygen in the raw material pitch for carbon fiber production is 1.0% by mass, preferably 1.5% by mass, and more preferably 1.7% by mass. On the other hand, the upper limit of the content ratio of oxygen is preferably 5.0% by mass, more preferably 4.0% by mass, and further preferably 3.0% by mass. In the case where the content ratio of oxygen is less than the above lower limit, the crystal growth on carbonization cannot be sufficiently suppressed and there is possibility that the resulting carbon fiber becomes liable to be torn apart due to stress concentration. Conversely, in the case where the content ratio of oxygen is more than the above upper limit, there is possibility that the production cost of carbon fiber rises because the mass reduction rate on carbonization is large and the yield of carbon fiber is lowered.

The lower limit of the content ratio of a toluene-soluble content in the raw material pitch for carbon fiber production is 20% by mass, preferably 30% by mass, and more preferably 35% by mass. On the other hand, the upper limit of the content ratio of a toluene-soluble content is preferably 80% by mass, more preferably 60% by mass, and further preferably 50% by mass. In the case where the content ratio of a toluene-soluble content is less than the above lower limit, there is possibility that the meltability and spinnability on melt spinning become insufficient. Conversely, in the case where the content ratio of a toluene-soluble content is more than the above upper limit, there is possibility that the production cost of carbon fiber rises because the yield of carbon fiber is lowered.

Examples of coal as a raw material of the raw material pitch for carbon fiber production include anthracite coal, bituminous coal, sub-bituminous coal, and brown coal, which are arranged in descending order of the degree of coalification, and the like, and of these, bituminous coal or sub-bituminous coal, which has a moderate degree of coalification, is preferred. Since bituminous coal and sub-bituminous coal have a relatively high content ratio of a toluene-soluble content and a moderate oxygen content ratio, by adopting bituminous coal or sub-bituminous coal as raw material coal, the yield of the raw material pitch for carbon fiber production having an oxygen content ratio and a toluene-soluble content ratio which are set so as to lie within the above-mentioned range can be made to become large. In this connection, brown coal being lower in the degree of coalification than sub-bituminous coal has a drawback that the yield of carbon fiber from raw material pitch becomes low because the oxygen content ratio is too large. Moreover, anthracite coal being high in the degree of coalification than bituminous coal has a drawback that raw material pitch is not easily subjected to melt spinning because the oxygen content ratio and the toluene-soluble content ratio are small.

[Method of Producing Raw Material Pitch for Carbon Fiber Production]

Subsequently, a method of producing the raw material pitch for carbon fiber production will be described.

The raw material pitch for carbon fiber production can be produced by the production method shown in FIG. 1. The method of producing raw material pitch for carbon fiber production shown in FIG. 1 includes the step of forming ash-free coal being a thermal decomposition product of coal by thermal decomposition of coal in a solvent and an extraction treatment (Thermal Decomposition Product Formation Step: Step S1), the step of separating the ash-free coal obtained in the thermal decomposition product formation step into a solvent-soluble component and solvent-insoluble component by a low-temperature solvent extraction treatment (Separation Step: Step S2), and the step of subjecting the solvent-soluble component obtained in the separation step to a heat treatment (Heat Treatment Step: Step S3).

<Thermal Decomposition Product Formation Step>

In the thermal decomposition product formation step of Step S1, slurry prepared by mixing raw material coal and a solvent is heated to a thermal decomposition temperature or more of the raw material coal, a solvent-soluble component of the thermally decomposed raw material coal is extracted with the solvent, and a solvent-insoluble component at the thermal decomposition temperature is removed to obtain ash-free coal. In this connection, “ash-free coal” refers to improved coal prepared by reforming a kind of coal and refers to one in which the content of an ash component (ash content) is 5% or less, preferably 3% or less, and more preferably 1% or less. In this connection, “the ash content” means a value measured in accordance with JIS-M8812 (2004).

The above-mentioned solvent is not particularly limited as long as the solvent has a property of dissolving raw material coal, and for example, monocyclic aromatic compounds such as benzene, toluene, and xylene, bicyclic aromatic compounds such as naphthalene, methylnaphthalene, dimethylnaphthalene, and trimethylnaphthalene, tricyclic aromatic compounds such as anthracene, and the like can be used. In this connection, examples of the above-mentioned bicyclic aromatic compound include a kind of naphthalene having an aliphatic chain and a kind of biphenyl having a long-chain aliphatic chain.

Even among the above-mentioned solvents, a bicyclic or tricyclic aromatic compound being a coal derivative prepared by refining a dry-distilled product of coal is preferred. The bicyclic aromatic compound being a coal derivative is stable even in a state of being heated and is excellent in affinity with coal. As such, by using such a bicyclic aromatic compound as a solvent, a proportion of a coal component extracted with the solvent can be heightened and the solvent can be easily recovered by a method such as distillation to be circularly used.

The lower limit of a heating temperature (thermal decomposition extraction temperature) of the slurry is preferably 300° C., more preferably 350° C., and further preferably 380° C. On the other hand, the upper limit of a heating temperature of the slurry is preferably 450° C., and more preferably 420° C. In the case where the heating temperature of the slurry is less than the above lower limit, since bonding between molecules constituting coal fails to be sufficiently weakened, for example, in the case of using low-grade coal as raw material coal, there are a fear that the resolidification temperature of the extracted ash-free coal fails to be heightened and a fear that the production process becomes uneconomical because the yield is lowered. Conversely, in the case where the heating temperature of the slurry is more than the above upper limit, there is possibility that the oxygen content ratio of ash-free coal becomes insufficient because the thermal decomposition reaction of coal is made very active and there is possibility that the extraction rate of ash-free coal is lowered because recombination of a radical generated by thermal decomposition occurs.

Although the extraction rate (yield of ash-free coal) from coal in the thermal decomposition product formation step depends on the quality of coal as a raw material, in the case of bituminous coal or sub-bituminous coal, for example, the extraction rate is 20% by mass or more and 60% by mass or less.

<Separation Step>

In the separation step of Step S2, by subjecting the ash-free coal obtained in the thermal decomposition product formation step of Step S1 to a low-temperature solvent extraction treatment, the ash-free coal is separated into a solvent-soluble component which is solvent-extracted at a low temperature and has a relatively low molecular weight and a solvent-insoluble component which is not solvent-extracted and has a relatively high molecular weight. With this setup, a melt-spinnable solvent-soluble component is obtained.

More specifically, crushed ash-free coal is dispersed in a solvent to prepare slurry, the temperature of the slurry is maintained within a prescribed temperature range over a certain period of time, and then, the slurry is separated into a solid content, namely, a solvent-insoluble component, and a liquid content, namely, a solution in which a solvent-soluble component is dissolved.

The lower limit of the average particle diameter of ash-free coal dispersed in a solvent is preferably 50 μm, and more preferably 100 μm. On the other hand, the upper limit of the average particle diameter of ash-free coal dispersed in a solvent is preferably 3 mm, and more preferably 1 mm. In the case where the average particle diameter of ash-free coal dispersed in a solvent is less than the above lower limit, there is possibility that separating slurry into a liquid containing the extracted solvent-soluble component and a solid content being a solvent-insoluble component becomes difficult. Conversely, in the case where the average particle diameter of ash-free coal dispersed in a solvent is more than the above upper limit, there is possibility that the extraction efficiency of a solvent-soluble component is lowered. In this connection, “the average particle diameter” means a particle diameter at which the volumetric integrated value becomes 50% in a particle size distribution measured by a laser diffraction scattering method.

The lower limit of the mixing rate of ash-free coal to the solvent in the above-mentioned slurry is preferably 3% by mass, and more preferably 5% by mass. On the other hand, the upper limit of the mixing rate of ash-free coal to the solvent is preferably 40% by mass, and more preferably 30% by mass. In the case where the mixing rate of ash-free coal to the solvent is less than the above lower limit, there is possibility that the production process becomes uneconomical because the production efficiency is lowered. Conversely, in the case where the mixing rate of ash-free coal to the solvent is more than the above upper limit, there is possibility that the handling of the slurry and the separation of the solvent-insoluble component therefrom become difficult.

A method of separating slurry into a solution in which a solvent-soluble component is dissolved and a solvent-insoluble component is not particularly limited and known separation methods such as a filtration method, a centrifuge separation method, and a gravitational setting method, and the combination of two methods among these can be adopted. Even among these, preferred is the combination of a centrifuge separation method and a filtration method which enables continuous operation for manipulating the fluid, requires low operation costs, is suitable for treating a large amount of the fluid, and enables the solvent-insoluble component to be securely removed.

By removing the solvent from a liquid (supernatant liquid) thus separated from the solvent-insoluble component, the solvent-soluble component of ash-free coal is separated and recovered, and by removing the solvent from a solid matter concentrated liquid, the solvent-insoluble component of ash-free coal is separated and recovered. A method of removing the solvent from each of the supernatant liquid and the solid matter concentrated liquid mentioned above is not particularly limited and a general distillation method, a general evaporation method, and the like can be used. In particular, it is preferred that the removal of the solvent from the solvent-insoluble component be carried out by distillation in order to recover and reuse the solvent.

The solvent used in the above-mentioned separation step needs only to be one that can dissolve a low molecular weight component of ash-free coal and ones that are the same as the solvents used for the above-mentioned thermal decomposition product formation step can be used. As the solvent for the separation step, of these, preferred is a solvent with which a sufficient extraction rate is attained at a low temperature, preferably at ordinary temperature, and examples of such a preferred solvent include pyridine, methylnaphthalene, tetrahydrofuran, anthracene, and the like.

With regard to the solvent extraction treatment temperature in the separation step, the optimum temperature varies with the kind of the solvent. However, in general, the solvent extraction treatment temperature is preferably less than 300° C., more preferably 200° C. or less, and further preferably 150° C. or less. On the other hand, although the lower limit of the solvent extraction treatment temperature is not particularly limited, the lower limit thereof is preferably an ordinary temperature, for example, 20° C. In the case where the solvent extraction treatment temperature is more than the above upper limit, there is possibility that the spinning efficiency is lowered at the time of melt spinning because the molecular weight of a solvent-soluble component to be extracted is made large and the softening temperature becomes too high. Conversely, in the case where the solvent extraction treatment temperature is less than the above lower limit, there is possibility that the cost rises unnecessarily because cooling is required.

The lower limit of the extraction time, namely, the time period during which the above-mentioned solvent extraction treatment temperature is maintained, in the separation step is preferably 10 minutes, and more preferably 15 minutes. On the other hand, the upper limit of the extraction time is preferably 120 minutes, and more preferably 90 minutes. In the case where the extraction time is less than the above lower limit, there is possibility that the low molecular weight component of ash-free coal fails to be sufficiently dissolved. Conversely, in the case where the extraction time is more than the above upper limit, there is possibility that the production cost increases unnecessarily.

The lower limit of the extraction rate of a solvent-soluble component from ash-free coal in the separation step is preferably 10% by mass, more preferably 20% by mass, and further preferably 30% by mass. On the other hand, the upper limit of the extraction rate of a solvent-soluble component from ash-free coal is preferably 90% by mass, more preferably 70% by mass, and further preferably 50% by mass. In the case where the extraction rate of a solvent-soluble component from ash-free coal in the separation step is less than the above lower limit, there is possibility that the production cost of the raw material pitch for carbon fiber production increases because the yield rate is lowered. Conversely, in the case where the extraction rate of a solvent-soluble component from ash-free coal in the separation step is more than the above upper limit, there is possibility that the spinning efficiency is lowered because the softening temperature of the solvent-soluble component becomes high.

<Heat Treatment Step>

In the heat treatment step of Step S3, by heating a solvent-soluble component obtained in the separation step of Step S2 to make the low molecular weight component volatilize and making a component thermally decomposed at a low temperature previously decompose to be removed, the raw material pitch for carbon fiber production is obtained. As such, by previously removing a volatile component and a decomposable component which may inhibit melt spinning, the raw material pitch for carbon fiber production is easily subjected to melt spinning and this enables carbon fiber excellent in tensile strength to be produced at relatively low costs.

With regard to the above-mentioned heat treatment, it is preferred that the solvent-soluble component be heated in a non-oxidizing gas atmosphere. As such, by heating the solvent-soluble component in a non-oxidizing gas atmosphere to prevent crosslinking by oxidation, inconveniences such as a rise in the softening temperature can be prevented. Although the above-mentioned non-oxidizing gas is not particularly limited as long as the gas can suppress the oxidation of pitch, a nitrogen gas is more preferred from the viewpoint of economy.

Moreover, it is preferred that the above-mentioned heat treatment be performed under the decompression condition. As such, by performing the heat treatment under the decompression condition, the vapor of a volatile component and the gaseous matter of a thermal decomposition product can be efficiently removed from pitch.

The lower limit of the heat treatment temperature in the above-mentioned heat treatment step is preferably 150° C., more preferably 170° C., and further preferably 200° C. On the other hand, the upper limit of the above-mentioned heat treatment temperature is preferably 350° C., more preferably 320° C., and further preferably 280° C. In the case where the above-mentioned heat treatment temperature is less than the above lower limit, there is possibility that the spinning efficiency is lowered because the volatile component in a solvent-insoluble component cannot be sufficiently removed and the spinability of the raw material pitch for carbon fiber production becomes insufficient. Conversely, in the case where the above-mentioned heat treatment temperature is more than the above upper limit, there are a fear that the energy cost increases unnecessarily, a fear that a useful component is thermally decomposed and the production efficiency of carbon fiber is lowered, and a fear that carbonization is made to further proceed and the spinnability is lowered.

Moreover, it is preferred that the heat treatment temperature in the heat treatment step be higher than the solvent extraction treatment temperature in the separation step of Step S2. As such, by making the heat treatment temperature higher than the solvent extraction treatment temperature, a volatile component having a boiling point higher than the solvent extraction treatment temperature can be removed from pitch. With this setup, by making the volatile component move out of the raw material pitch for carbon fiber production at the time of spinning, the formation of a pore and the breakage of filament yarn can be prevented.

Moreover, it is preferred that the heat treatment temperature in the heat treatment step be higher than the melt spinning temperature. As such, by making the heat treatment temperature higher than the melt spinning temperature, in this heat treatment step, a component that can thermally decompose at the time of melt spinning can be made to previously thermally decompose to be removed. With this setup, the breakage of filament yarn, being obtained by spinning pitch into yarn, which is caused by a thermal decomposition product generated at the time of spinning and the formation of a defect in the finally resultant carbon fiber which is caused by the thermal decomposition product can be prevented.

The lower limit of the heat treatment time (the time period during which the temperature is maintained at the above-mentioned heat treatment temperature) in the above-mentioned heat treatment step is preferably 10 minutes, and more preferably 15 minutes. On the other hand, the upper limit of the heat treatment time in the above-mentioned heat treatment step is preferably 120 minutes, and more preferably 90 minutes. In the case where the heat treatment time in the above-mentioned heat treatment step is less than the above lower limit, there is possibility that the low molecular weight component fails to be sufficiently removed. Conversely, in the case where the heat treatment time in the above-mentioned heat treatment step is more than the above upper limit, there is possibility that the treatment cost increases unnecessarily.

The lower limit of the softening temperature of the raw material pitch for carbon fiber production which is obtained by subjecting the solvent-soluble component to a heat treatment is preferably 150° C., and more preferably 170° C. On the other hand, the upper limit of the softening temperature of the raw material pitch for carbon fiber production is preferably 280° C., and more preferably 250° C. In the case where the softening temperature of the raw material pitch for carbon fiber production is less than the above lower limit, there is possibility that the infusibilization treatment becomes inefficient because the infusibilization treatment temperature cannot be made high. Conversely, in the case where the softening temperature of the raw material pitch for carbon fiber production is more than the above upper limit, the melt spinning temperature needs to be made high and there is possibility that spinning becomes unstable and a fear that the cost increases. In this connection, “the softening temperature” refers to a value measured by a ring and ball method in accordance with ASTM-D36.

The lower limit of the yield of the raw material pitch for carbon fiber production from the solvent-soluble component obtained in the above-mentioned separation step in this heat treatment step is preferably 80% by mass, and more preferably 85% by mass. On the other hand, the upper limit of the yield of the raw material pitch for carbon fiber production from the solvent-soluble component in the heat treatment step is preferably 98% by mass, and more preferably 96% by mass. In the case where the yield of the raw material pitch for carbon fiber production from the solvent-soluble component in the heat treatment step is less than the above lower limit, there is possibility that the yield rate of the raw material pitch for carbon fiber production is unnecessarily lowered. Conversely, in the case where the yield of the raw material pitch for carbon fiber production from the solvent-soluble component in the heat treatment step is more than the above upper limit, there is possibility that the spinning efficiency is lowered because the spinability of pitch becomes insufficient due to residual components such as a volatile component and a component thermally decomposed at a low temperature in the raw material pitch for carbon fiber production.

[Method of Producing Carbon Fiber]

Furthermore, a method of producing carbon fiber with the use of the raw material pitch for carbon fiber production will be described.

The method of producing carbon fiber with the use of the raw material pitch for carbon fiber production includes a process of subjecting the raw material pitch for carbon fiber production to melt spinning, a process of subjecting filament yarn obtained by the melt spinning to infusibilization, and a process of subjecting the infusibilized filament yarn to carbonization.

<Melt Spinning Process>

In the melt spinning process, using a known spinning apparatus, the raw material pitch for carbon fiber production is subjected to melt spinning. That is, raw material pitch in a melted state is made to pass through a nozzle (spinneret) to be formed in a filament-form and the raw material pitch is cooled so that the shape thereof is fixed in the filament-form.

As the nozzle used in the melt spinning, a known one may be used and a nozzle having a diameter of 0.1 mm or more and 0.5 mm or less and a length of 0.2 mm or more and mm or less can be used. For example, filament yarn obtained by melt spinning raw material pitch into yarn is wound around a drum with a diameter of 100 mm or more and 300 mm or less or so.

The lower limit of the melt spinning temperature is preferably 180° C., and more preferably 200° C. On the other hand, the upper limit of the melt spinning temperature is preferably 350° C., and more preferably 300° C. In the case where the melt spinning temperature is less than the above lower limit, there is possibility that stable spinning fails to be carried out because the raw material pitch is insufficiently melted. Conversely, in the case where the melt spinning temperature is more than the above upper limit, there is possibility that filament yarn obtained by spinning raw material pitch into yarn is broken because a component in the raw material pitch is thermally decomposed.

Although the lower limit of linear velocity at the time of melt spinning is not particularly limited, the lower limit of the linear velocity is preferably 100 m/min, and more preferably 150 m/min. On the other hand, the upper limit of linear velocity at the time of melt spinning is preferably 500 m/min, and more preferably 400 m/min. In the case where the linear velocity at the time of melt spinning is less than the above lower limit, there is possibility that carbon fiber becomes expensive because the production efficiency is lowered. Conversely, in the case where the linear velocity at the time of melt spinning is more than the above upper limit, the production efficiency is rather lowered because spinning becomes unstable and there is still a fear that carbon fiber becomes expensive.

The lower limit of the average diameter of filament yarn obtained by being spun into yarn in the melt spinning process is preferably 5 μm, and more preferably 7 μm. On the other hand, the upper limit of the average diameter of filament yarn obtained by being spun into yarn in the melt spinning process is preferably 20 μm, and more preferably 15 μm. In the case where the average diameter of filament yarn is less than the above lower limit, there is possibility that spinning fails to be stably carried out. Conversely, in the case where the average diameter of filament yarn is more than the above upper limit, there is possibility that the flexibility of filament yarn becomes insufficient.

<Infusibilization Process>

In the infusibilization process, filament yarn obtained in the melt spinning process is heated in an atmosphere containing oxygen to be crosslinked and infusibilized. As the atmosphere containing oxygen, air is generally adopted.

The lower limit of the infusibilization treatment temperature is preferably 150° C., and more preferably 200° C. On the other hand, the upper limit of the infusibilization treatment temperature is preferably 300° C., and more preferably 280° C. In the case where the infusibilization treatment temperature is less than the above lower limit, there are a fear that infusibilization becomes insufficient and a fear that the infusibilization treatment becomes inefficient because the infusibilization treatment time is prolonged. Conversely, in the case where the infusibilization treatment temperature is more than the above upper limit, there is possibility that filament yarn melts before being aerobically crosslinked.

The lower limit of the infusibilization treatment time is preferably 10 minutes, and more preferably 20 minutes. On the other hand, the upper limit of the infusibilization treatment time is preferably 120 minutes, and more preferably 90 minutes. In the case where the infusibilization treatment time is less than the above lower limit, there is possibility that infusibilization becomes insufficient. Conversely, in the case where the infusibilization treatment time is more than the above upper limit, there is possibility that the production cost of carbon fiber increases unnecessarily.

<Carbonization Process>

In the carbonization process, the filament yarn infusibilized in the infusibilization process is heated and carbonized to obtain carbon fiber.

Specifically, filament yarn is installed in any heating apparatus such as an electric furnace and the inside thereof is replaced with a non-oxidizing gas, after which the filament yarn is heated while a non-oxidizing gas is blown into the heating apparatus.

The lower limit of the heat treatment temperature in the carbonization process is preferably 700° C., and more preferably 800° C. On the other hand, the upper limit of the heat treatment temperature is preferably 3000° C., and more preferably 2800° C. In the case where the heat treatment temperature is less than the above lower limit, there is possibility that carbonization becomes insufficient. Conversely, in the case where the heat treatment temperature is more than the above upper limit, there is possibility that the production cost rises from the viewpoints of the enhancement in heat resistance of facilities and the fuel consumption.

The heating time in the carbonization process also needs only to be appropriately set according to the characteristics required for a carbon material. Although the heating time is not particularly restricted, the heating time is preferably 15 minutes or more and 10 hours or less. In the case where the heating time is less than the above lower limit, there is possibility that carbonization becomes insufficient. Conversely, in the case where the heating time is more than the above upper limit, there is possibility that the productive efficiency of a carbon material is lowered.

Although the above-mentioned non-oxidizing gas is not particularly limited as long as the oxidation of a carbon material is suppressed, nitrogen gas is preferred from the viewpoint of economy.

Other Embodiments

The configuration of the present invention should not be limited by the above-mentioned embodiments. Accordingly, based on the description in the present specification and the common general technical knowledge, the omission, replacement, and addition for constituent elements in the respective parts of the above-mentioned embodiments are possible and all of those should be contemplated within the present invention.

As an example, in a method of producing the raw material pitch for carbon fiber production, without separating an ash content and the like in slurry from ash-free coal in the thermal decomposition product formation step, the ash content and the like may be separated together with a solvent-insoluble component in ash-free coal in the subsequent separation step.

Moreover, in a method of producing the raw material pitch for carbon fiber production, the heat treatment step may be omitted.

EXAMPLES

Hereinafter, the present invention will be described in detail on the basis of examples, but the present invention should not be restrictively construed on the basis of the description of these examples.

<Raw Material Pitch for Carbon Fiber Production>

As described below, in Examples 1 to 6 and Comparative Examples 1 and 2, respective kinds of raw material pitch for carbon fiber production which are different in production condition were experimentally produced and subjected to melt spinning, infusibilization, and carbonization under the same condition to experimentally produce respective kinds of carbon fiber.

Example 1

As raw material coal, Australian bituminous coal having an oxygen content ratio of 6.5% by mass in terms of anhydrous ash-free coal was used. First, the above-mentioned bituminous coal was crushed into a size of 1 mm or less, 1 kg of the bituminous coal was mixed with 5 kg of methylnaphthalene, the mixture was placed in an autoclave, the internal atmosphere thereof was replaced with nitrogen, and the internal temperature thereof was held at 400° C. for 1 hour and then decreased with cooling to obtain a thermal decomposition product. Next, to this thermal decomposition product, 5 kg of methylnaphthalene was further added, the contents were stirred for 1 hour at an extraction temperature of 60° C. to extract a methylnaphthalene-soluble component and filtered, and the obtained filtrate was distilled under reduced pressure to separate the methylnaphthalene-soluble component. This methylnaphthalene-soluble component was subjected to a heat treatment for 1 hour under a nitrogen atmosphere at a heat treatment temperature of 230° C. to obtain raw material pitch for carbon fiber production of Example 1.

Examples 2 to 6

In Example 2, raw material pitch was experimentally produced in the same manner as that in Example 1 except that the extraction temperature was set to 80° C. In Example 3, raw material pitch was experimentally produced in the same manner as that in Example 1 except that the extraction temperature was set to 100° C. In Example 4, raw material pitch was experimentally produced in the same manner as that in Example 1 except that the heat treatment temperature was set to 250° C. In Example 5, raw material pitch was experimentally produced in the same manner as that in Example 1 except that the extraction temperature was set to 80° C. and the heat treatment temperature was set to 250° C. In Example 6, raw material pitch was experimentally produced in the same manner as that in Example 1 except that the extraction temperature was set to 100° C. and the heat treatment temperature was set to 250° C.

Comparative Example 1

As in the case of Example 1, bituminous coal was crushed into a size of 1 mm or less, 1 kg of the bituminous coal was mixed with 5 kg of methylnaphthalene, the mixture was placed in an autoclave, the internal atmosphere thereof was replaced with nitrogen, and the internal temperature thereof was held at 400° C. for 1 hour, the contents were filtered to obtain ash-free coal, and the ash-free coal was subjected to a heat treatment for 1 hour under a nitrogen atmosphere at a heat treatment temperature of 200° C. to obtain raw material pitch for carbon fiber production of Comparative Example 1.

Comparative Example 2

With regard to raw material pitch for carbon fiber production of Comparative Example 2, commercially available hard pitch having an oxygen content of 0.9% by mass and a toluene-soluble content of 64% by mass was prepared and the hard pitch was subjected to a heat treatment for 20 hours under a nitrogen atmosphere at a heat treatment temperature of 350° C. to obtain raw material pitch for carbon fiber production of Comparative Example 2.

<Component Analysis>

The respective kinds of raw material pitch for carbon fiber production of Examples 1 to 6 and Comparative Examples 1 and 2 were measured for the oxygen content ratio in accordance with JIS-M8813 (2004). Moreover, the respective kinds of raw material pitch for carbon fiber production of Examples 1 to 6 and Comparative Examples and 2 were measured for the toluene-soluble content ratio in accordance with JIS-K2207 (1996).

<Carbon Fiber>

With regard to experimental production of carbon fiber with the use of the respective kinds of raw material pitch for carbon fiber production of Examples 1 to 6 and Comparative Examples 1 and 2, first, in a spinning machine having a nozzle of a diameter of 0.2 mm and a length of 0.4 mm, pitch to be spun into yarn was placed and subjected to melt spinning at 250° C. On this occasion, filament yarn obtained by spinning pitch into yarn was wound around a drum of a diameter of 100 mm which was rotated at 600 rpm (linear velocity of about 190 m/min). Subsequently, the filament yarn was heated for 1 hour at 250° C. in the air to be infusibilized. Furthermore, the insolubilized fiber was carbonized at 800° C. In this connection, in Comparative Example 1, carbon fiber failed to be obtained because raw material pitch for carbon fiber production of Comparative Example 1 failed to be stably subjected to melt spinning even when heated to 350° C.

<Tensile Strength>

Respective kinds of carbon fiber experimentally produced with the use of the respective kinds of raw material pitch for carbon fiber production of Examples 1 to 6 and Comparative Examples 1 and 2 were measured for the tensile strength in accordance with JIS-L1013 (2010).

The oxygen content ratio of the respective kinds of raw material pitch for carbon fiber production of Examples 1 to 6 and Comparative Examples 1 and 2, the toluene-soluble content ratio thereof, the yield from the raw material coal (hard pitch in Comparative Example 2) in terms of anhydrous ash-free coal, and the tensile strength of carbon fiber experimentally produced with the use thereof are shown in the following Table 1.

TABLE 1 Toluene- Oxygen soluble Content Content Tensile Ratio Ratio Yield Strength (% by mass) (% by mass) (% by mass) (MPa) Example 1 2.2 56 26 1,100 Example 2 1.9 42 30 1,200 Example 3 1.8 36 32 1,150 Example 4 1.4 31 26 1,150 Example 5 1.7 28 28 1,000 Example 6 1.7 33 32 900 Comparative 1.8 17 46 — Example 1 Comparative 0.6 19 (92) 550 Example 2

As such, it was confirmed that, by making raw material pitch for carbon fiber production have a content ratio of oxygen of 1.0% by mass or more and a content ratio of a toluene-soluble content of 20% by mass or more, carbon fiber relatively excellent in tensile strength can be stably produced.

INDUSTRIAL APPLICABILITY

The raw material pitch for carbon fiber production of the present invention is suitably utilized in the production of carbon fiber.

DESCRIPTION OF REFERENCE SIGNS

-   -   S1: Thermal decomposition product formation step     -   S2: Separation step     -   S3: Heat treatment step 

1: A raw material pitch, comprising oxygen in a content of 1.0% by mass or more and a toluene-soluble component in a content of 20% by mass or more, wherein the raw material pitch is obtained from coal and is suitable for producing carbon fiber by melt spinning. 2: The raw material pitch according to claim 1, wherein the coal is bituminous coal or sub-bituminous coal. 3: The raw material pitch according to claim 1, which is prepared by a process comprising subjecting a solvent-soluble component separated from a thermal decomposition product of coal in a solvent by a solvent extraction treatment at a temperature of less than 300° C. to a heat treatment at a temperature of 150° C. or more. 4: The raw material pitch according to claim 2, which is prepared by a process comprising subjecting a solvent-soluble component separated from a thermal decomposition produce of coal in a solvent by a solvent extraction treatment at a temperature of less than 300° C. to a heat treatment at a temperature of 150° C. or more. 